In effect, in the production of pipes by thermoplastic extrusion designed for making conduits for delivering and/or discharging fluids (used for example in the drainage networks, drinking water distribution networks and sewers of building works), the belling machines are used for forming an end portion of the pipes into the characteristic “bell” shape. This particular wider shape is used to connect the pipes in succession which form a conduit. An unshaped end of a pipe is normally inserted in the bell-shaped end of the adjacent pipe in the conduit.
The belling machine is normally installed along the extrusion line where it receives the cut pipes to be processed.
The majority of belling machines make the bell with the hot forming process. The belling machines are equipped with at least one oven which heats the end of the pipe, changing the wall of the end of the pipe to be shaped into a plastically deformable softened state. The machine is further equipped with forming equipment which, by using a suitable mould, forms the heated end of the pipe into a bell shape. The bell shaped on the mould is generally cooled inside the same forming equipment.
The most commonly used thermoplastic materials in pipe systems are unplasticised polyvinyl chloride (PVC-U), polypropylene (PP) and high-density polyethylene (HDPE).
The state of optimum plastic deformability and therefore the final thermal state of the pipe before the belling process depends on the material, the shape of the bell, the wall thickness of the pipe, the dimensions specified for the bell and the characteristics of the forming and/cooling method.
The PVC-U thermoplastic material is a substantially amorphous material which at ambient temperature exhibits a fragile mechanical behaviour similar to glass whilst at temperatures greater than 75° C.-80° C. (vitreous transition temperature Tg) it starts to soften, exhibiting a plastic rubbery behaviour. Normally, with the PVC-U pipe the belling step allows a relatively large optimum thermal heat state range.
If therefore, the optimum temperatures for a belling process of the PVC-U pipe are approximately between 90° C. and 125° C., the same cannot be said for the other above-mentioned materials, polypropylene (PP) and high-density polyethylene (HDPE).
Polypropylene (PP) and high-density polyethylene (HDPE) at ambient temperature are, in effect, semi-crystalline, that is to say, there coexists in them an ordered macromolecular crystalline configuration and a disordered amorphous macromolecular configuration.
At ambient temperature the amorphous part is in a viscous liquid state, so the ambient temperature is greater than the vitreous transition temperature of the amorphous part of the material.
At ambient temperature, PP and HDPE exhibit a ductile and tough mechanical behaviour.
Unlike PVC-U, both PP and HDPE have a melting temperature, that is to say, a temperature above which there is a destruction of the crystalline mesh and the change of state from solid to liquid. The melting temperature of polypropylene (PP) is approximately 165° C., whilst that of high-density polyethylene (HDPE) is approximately 134° C.
The change from the solid state to the liquid state is therefore quite sudden and occurs in a very small temperature range equal to approximately ±1° C. relative to the melting temperature.
At temperatures higher than melting temperature the process of forming the bell in the hot state cannot be performed. For this reason, the optimum thermal state for the belling of PP and HDPE pipes is certainly less than the melting temperature.
Consequently, the shaping of the bell in PP and HDPE pipes occurs with material which exhibits a viscoelastic and viscoplastic mechanical behaviour, so, unlike PVC-U pipes, the elastic behaviour of the material in the mechanical shaping deformation is not negligible. With temperatures which are too low the plastic deformability of the material can be insufficient to form the shape of the bell, or, even if it can be formed, the resulting bell is subject to shrinkage phenomena due to the memory of the initial dimensions of the pipe, the shrinkage effects increasing the greater is the elastic part of the deformation made during the forming step.
For this reason, in the belling process for PP and HDPE pipes, variations are permitted in the hot working temperature which fall within a very small range, approximately ±2° C. around the predetermined optimum value.
Therefore, compared with PVC-U pipes, the most common belling processes applied to PP and HDPE pipes require a temperature distribution along the axis of the pipe and in the thickness of the wall which is almost uniform, as with different temperatures in various zones of the pipe there would be a different behaviour with the shrinkage of the various zones of the bell and, as a direct consequence, an unacceptable distortion in shape and a dimensional instability of the bell itself.
In order to limit at least partly the occurrence of these unwanted circumstances, contact heating ovens are widely applied in the belling machines for PP and HDPE pipes. The contact ovens are substantially configured with metallic masses which adhere to the surface of the end of the pipe. The metallic masses are maintained at a precise temperature and transfer the heat to the wall of the pipe by conduction. Normally, the contact oven is configured with shells which enclose the outside of the wall of the pipe. A spindle which heats by contact or a device which operates with a different heat transmission system is inserted inside the pipe in the same oven.
Whilst it is relatively simple to heat the outer surface of the pipe, it is much more complex to achieve a heating by contact of the inner surface. In effect, due to the features of the pipe extrusion process, the wall thickness and, therefore, the inside diameter of the pipe is never as regular as the outside diameter; it follows that the internal heating element can if anything be close the inner wall, but not adhere to the surface. Moreover, the reduced space available inside the small diameter pipes (the commercial diameters currently start from 32 mm) makes it complex and costly to make an efficient internal heating system.
For these reasons it is normally preferred to heat the pipe mainly by external contact, adopting solutions for the internal heating that make the heating process faster and such as not create thermal discontinuities in the wall of the pipe.
For example, the internal heating is performed with systems which mainly heat by radiation with the use of measures, typically rotary devices, such as to render the heating uniform in the circumference of the pipe.
The heating which occurs mainly by external contact, with transmission of heat by conduction, is in any case a slow heating process.
In the belling machines for PP and HDPE pipes such as those described in patent documents IT 1 171 936 and EP 700 771, the time necessary for forming and cooling the bell are considerably shorter than the times required for heating the end of the pipe prior to shaping.
Purely by way of example, to form the bell in a common PP pipe for drains of buildings (outside diameter 110 mm, wall thickness 2.7 mm) a forming-cooling time of approximately 15 seconds is required, whilst the heating with contact systems makes the pipe formable in a time of not less than 45 seconds.
For this reason, the prior art belling machines for PP and HDPE pipes are normally configured with a single forming-cooling station associated with a plurality of heating stations.
Therefore, the cut pipe coming from the extrusion line is heated, at the end to be formed into a bell shape, in the various heating stations, before being inserted in the single forming and cooling station.
The need to produce short pipes and at the same time maintain high extrusion speeds results in the need for high production belling machines; for this reason the systems for belling drain pipes for buildings are suitably configured to operate in a multi-belling mode.
It has been seen that a precise heating is not required for PVC-U pipes, unlike for PP and HDPE pipes and, therefore, hot air ovens or ovens with radiation heating elements can be conveniently used which are able to heat in an optimum manner, the PVC-U pipes, in much shorter times than that which can be achieved with contact ovens.
For example, belling machines for PVC-U pipes configured with a single heating station equipped with a hot air oven, even in multi-belling mode, are able to support the same productivity as belling machines configured with three or four contact stations. The hot air or radiation ovens are also usually less complex and expensive than contact ovens for multi-belling. In effect, these belling machines configured with radiation or hot air ovens can only be used for PVC-U pipes, where, on the contrary, the greatest demand for drain pipes for buildings is currently for PP and HDPE pipes.
To overcome these limitations, at least partly, “hybrid” machines are made where the first heating station is of the hot air or radiation type, whilst the second and last station is of the contact type. In this way, a fast, but imprecise, pre-heating is achieved with the first station, whilst the second and last contact station completes the heating bringing the end of the pipe to the precise and uniform heating necessary for PP and HDPE pipes.
Amongst the radiant ovens used in belling machines, the so-called short wave radiant ovens are important, such as that described in patent document DE10058505, as it makes it possible to obtain reduced, and therefore advantageous, heating times. This device basically comprises a container limited by metallic walls open at the side where the end of the pipe to be heated enters. The inside of the container houses several electrically powered heating elements.
The apparatus is equipped with heating elements, so-called radiation units, which transfer electromagnetic energy in the form of infra-red rays, the radiation emitted mainly in the short wave range (0.9 μm-1.6 μm). Compared with the more common long and medium wave radiation units, used in the belling machines for PVC-U pipes, the short wave radiation units have a greater penetration capacity.
The short wave radiation units, commonly known also as infra-red lamps, are rectilinear tubes made of quartz glass, inside of which a tungsten wire is located which reaches temperatures in the order of 2000° C. These lamps have very short starting times, of less than 2 seconds, a feature which makes it possible to keep the radiation units de-energised during the phases in which the pipe is not present in the oven, with obvious energy savings.
In the short wave radiation ovens a certain number of lamps are positioned parallel to the axis of the pipe.
Since the distribution of the radiation is not uniform around the pipe in the oven, the pipe is maintained in constant rotation during the heating step in order to obtain a uniform heating along the circumference.
With a focussed positioning and selection of the radiation units, in terms of number and power, it is possible to obtain very short heating times providing the radiation units are installed both outside and inside the pipe.
The two-sided heating is also necessary to obtain the maximum uniformity of temperature in the thickness of the wall.
However, in the belling of PP and HDPE pipes a uniform heating is also necessary along the axis of the pipe and the configuration of the oven described above does not favour this type of heating. In effect, given the laws of the transmission of heat by radiation, a pipe exposed to a radiation unit positioned parallel to its axis is heated more in the zone which faces the central part of the emitter, since this pipe zone is overall closest to the heating system; with the heating reducing in the zones of the pipe which face the ends of the heating element. This problem can be solved, at least for the outer side of the pipe, by preparing the oven with at least two groups of lamps suitably offset along the direction of the axis of the pipe. This arrangement, together with the use of suitable metallic screens close to the access opening of the oven, allows the creation, by superposition of the effects, of a substantially uniform heating area.
However, it is difficult to adopt a similar solution to make the heating of the inner side of the pipe uniform, also because, especially inside small diameter pipes, there is no space available to insert two groups of lamps, of suitable power, conveniently offset parallel to the axis of the pipe.